Plants thrive on varying levels of soil moisture

by Nancy O'Donnell - Sept. 12, 2005 12:00 AMAlbany Times Union

Every plant has a preference when it comes soil moisture. Tags will tell you the plant prefers a "moist" soil, or a "well-drained" soil, some even a "dry" soil, but how do you know what moist is and how is moisture measured?

Soil moisture is the amount of water present in the soil surrounding the roots on a continual basis; not right after a heavy rain or a spring snow melt, but rather on a day-to-day, month-to-month average. It varies greatly due to soil type, amount of organic matter present, exposure to sunlight and temperature.

But what doesn't vary is the plant's dependence on this moisture; not only do the roots take up water, but important minerals which have become dissolved in the water.

Thus the plant uses soil moisture as a means of transporting necessary minerals into and throughout its vascular system.

We know water is critical for photosynthesis, the plant's food-making process. We also know moisture affects seed germination. Some seeds require moisture to help break down their seed coat so germination can occur; morning glory seeds must be soaked in water for 24 hours prior to planting. Others require moisture for survival immediately following germination, such as the spores of a fern. Think about the environment you will find ferns thriving in; it's continually moist.

How does your soil rate when it comes to holding moisture? Soil labs often determine the moisture capacity of a sample by weighing it, heating it until the moisture has evaporated and then re-weighing; the difference is calculated as the percent of soil moisture. For homeowners, a simpler technique known as the "big squeeze" works well. Dig into the soil about 6 inches, grab a handful and squeeze.

If it remains in a "ball" and a wet outline of water appears in your palm, your soil is considered very moist. Should the "ball" break apart but remain in large clumps, your soil is moist.

A well-drained soil will "ball" and then crumble apart. A dry soil, depending on the soil type, will either flow through your fingers like sand through an hourglass or be rock hard and difficult to chip out of the ground.

As mentioned above, many other factors contribute to a soil's moisture-holding capacity, but one thing we as gardeners can do to help our soil is to add organic matter and lots of it.

Sandy soil is made up of larger soil particles than a clay soil. As a result, when rain falls it leaches or drains through the sand very quickly, thus sand is noted as having a poor moisture-holding capacity.

Clay, with its very tiny soil particles, is able to hold the initial rainfall for a longer period of time than sand, but when it dries out it is rock-hard. Organic matter also known as decomposing plant and animal debris such as straw, grass clippings, peat moss, leaves, compost and manure are able to hold 20 times their weight in water.

By adding organic matter to our soils, we not only add nutrients but help the soil retain moisture. Over time, if we add organic matter on an annual basis, we will begin to notice the quick drain of sandy soil beginning to slow and the fine particles of clay soils beginning to mix with the larger pieces of decaying matter. Soils high in organic matter are known as loam.

When it comes to hanging baskets, professional growers use a potting mix formulated for moisture retention. This is an example we should follow when filling our window boxes and planters. But because these planters and baskets are exposed to the sun's drying rays, keeping them moist can be a chore.

During the past few years, a product has come on the market that can be added to the mix to help retain moisture. It's similar to the filling you find in disposable diapers and can be found under various trade names at local home and garden centers, one being Soil Moist.

As little as a half-teaspoon of these tiny round beads, depending on the container size are added to the potting mix. When watered, they swell and turn into little gelatin globs that can hold excess moisture until the soil's moisture level drops, then they release the water they're holding.

Here are a few other plant survival tips for the summer heat:

• Wilted plants in containers should be immersed in a bucket of water until air bubbles stop rising. Often the pots become so root-bound or the potting mix too dry to absorb water from above.

• If leaving for a long weekend, place planters in a shady location and water thoroughly just before leaving. For longer jaunts, ask a friend to tend to them in your absence.

• Finally, a 3-inch layer of mulch works wonders in helping to retain soil moisture by interfering with evaporation and controlling soil temperature fluctuations.